This invention is in the field of electrostatic xerography and relates particularly to the application of toner to a receptor member.
In electrostatic xerography what is known as "spaced touchdown" donor development has a number of appealing characteristics. Among these are the potential for developing clean images at high speed the elimination of carrier beads from the developer and the reduction in the amount of developer required to be transported. In the absence of carrier beads, the associated bulk, mixing and developer life problems are avoided. In its simplest form, a touchdown development system consists of a donor surface on to which the developer (toner) is attached, electrostatically charged, and then transported to, and through the development zone, usually at a speed synchronous with the image bearing surface being developed.
In the past, several means have been utilized to attach toner to a donor means in spaced touchdown development method. Such means include adhesive, electrostatic, or mechanical force. Charging by means of a corona discharge device is the most common method of imparting a charge to toner but contact charging has also been employed.
A donor can be brought into contact with the surface bearing the electrostatic latent image during development but the preferred technique is to bring the donor into close proximity to the photoreceptor, usually at a distance of from about 0.005 inch to about 0.010 inch. Since any toner must be pulled across the gap by the image field, the background regions will be kept exceptionally free of spurious toner deposits. Maintaining a gap of from about 0.005 inch to about 0.010 inch between the donor and the image bearing surface is a difficult problem due to the mechanical tolerances of the donor system's components. Any variation in the thickness of the toner layer on the donor tend to aggravate this problem.
In the past, in order to keep these variations within acceptable limits it has often been necessary to level the toner with a doctor blade. However, a doctor blade has the undesirable feature that it can compact the toner to the point where its mobility in the electric development field is impaired. Encapsulated toner which is fragile or easily ruptured may also be crushed or fractured during the leveling operation. Since some toner compaction is inevitable during charging and development, it is often desirable to remove all toner and renew the surface of the donor with fresh toner after each cycle. (If the donor is of the adhesive type, it is usually discarded with the unused toner.) Donors which employ electrostatic force to hold the toner have recycling problems because it is difficult to completely neutralize the residual charge on the donor surface beneath the toner. Another problem in the employment of electrostatic force to retain toner on a donor, is the delicate balance which must be maintained to insure consistent toner adhesion and transport in cyclic operation. This balance has proven to be difficult to achieve in practice.
An alternative method of bonding toner to the donor, and one that has the advantage that the nature of the bonding force is different from the development forces, is to use a magnetized donor surface and a magnetically attractable toner. Toner can be made magnetically tractable if it is impregnated with a ferrous material such as iron oxide. The magnetic donor development schemes suggested in the past have been impractical for several different reasons. For example, it has been proposed that a magnetized donor member be employed to hold magnetic tonor thereon and to move that toner to a receptor member having a latent electrostatic image thereon. However, in all such previous proposals, the magnetic field pattern on the donors have been arranged to cause the toner to stand outwardly therefrom in the form of bristles of a brush. Such bristles or brush-like formations were wiped over the receptor member and the charged image areas thereon would attract and hold the toner material. However, those upstanding formations of the toner brushed over the entire surface of the receptor often result in darkened or smudged background areas.